The invention relates to a directly heated cathode for electron tubes, particularly transmitting tubes, with coaxial design of the electrodes and their lead-ins, which cathode exhibits a hollow cylinder which is secured at one end to an annular cathode lead, and at the other end is secured to a cathode cap which is mounted at a power supply lead extending coaxially in the hollow cylinder.
Such a cathode is known, for example, from the German AS No. 24 15 384. It further proceeds as known from this German AS that one uses tungsten wire as carrier for the manufacture of cathodes which are directly heated and exhibit a metal film as the emission layer, for example tungsten carbide with thorium film. The required surface and mechanical stability is achieved by means of an appropriate arrangement of the wires, in mesh or needle form in transmitting tubes. An inhomogeneous cathode surface which has effects on the electrode system follows of necessity from the design of the cathodes. There are also difficulties in the manufacture of cathodes with narrow wire intervals. In transmitting tubes with narrow grid-cathode intervals and meshed cathodes, the different intervals (outer wire and inner wire of the meshed cathode) have a deletorious effect on the electrical performance of the electron tubes.
Cathodes with a homogeneous surface are known up to now only as oxide matrix or dispenser cathodes. Specific operating relationships of electron tubes, particularly transmitting tubes of greater output, forbid the employment of such cathodes.